The best Porsche for each track is kinda class/track dependent, but here's some pointers from someone who adores them:
The 914/6 is decent for twisty tracks in C & D classes. Anything above those classes and you're better off in a 911.
The 1973 911 Carrera RS is as fast as the Dino or even any other Porsche on just about any track in B & A classes.
The 1982 911T is just as quick in A-Class as the 1973 Carrera RS on twisty tracks. It weighs more, but can fit wider tires.
F-Dog has a good point about other people being able to ruin your day if you're piloting the 911 in traffic, but it must also be said that the 911 has a very distinct advantage off the starting line in most multi-player rooms. It sucks if people hit you, but they have to catch you first!
The other P-cars, such as the Cayman and Boxster, are proven platforms. You can't go wrong in them in A-Class or even B-Class on some tracks... but wouldn't you rather be in a 911?
In S-Class, things start to get silly. I've got a build posted in the FCT garage for a 911 GT2 with a 959 drivetrain swap that's faster off the line than just about anything in the game. It weighs less than a comparable 959 and IIRC can fit wider tires! Skip the Carrera GT, it's just not that good at anything IMHO.
